1. Field of the Invention
The present invention relates data communications, and more particularly, efficiency in the transmission of data across a SONET network.
2. Description of the Related Art
A data communications network is the interconnection of two or more communicating entities (i.e., data sources and/or sinks) over one or more data links. A data communications network allows communication between multiple communicating entities over one or more data communications links. High bandwidth applications supported by these networks include streaming video, streaming audio, and large aggregations of voice traffic. In the future, the demands for high bandwidth communications are certain to increase. To meet such demands, an increasingly popular alternative is the use of lightwave communications carried over fiber optic cables. The use of lightwave communications provides several benefits, including high bandwidth, ease of installation, and capacity for future growth.
The synchronous optical network (SONET) protocol is among several protocols designed to employ an optical infrastructure. SONET is widely employed in voice and data communications networks. SONET is a physical transmission vehicle capable of transmission speeds in the multi-gigabit range, and is defined by a set of electrical as well as optical standards. A similar standard to SONET is the Synchronous Digital Hierarchy (SDH) which is the optical fiber standard predominantly used in Europe. There are only minor differences between the two standards. Accordingly, hereinafter any reference to the term SONET refers to both SDH and SONET networks, unless otherwise noted.
SONET utilizes a byte interleaved multiplexing scheme. Multiplexing enables one physical medium to carry multiple signals. Byte interleaving simplifies multiplexing and offers end-to-end network management. See Bellcore Generic Requirements document GR-253-CORE (Issue 2, December 1995), hereinafter referred to as the “SONET Specification,” and incorporated herein by reference for all purposes. The first step in the SONET multiplexing process involves the generation of the lowest level or base signal. In SONET, this base signal is referred to as synchronous transport signal-level 1, or simply STS-1, which operates at 51.84 Mbps (Megabits per second). Data between adjacent nodes is transmitted in these STS modules. Each STS is transmitted on a link at regular time intervals (for example, 125 microseconds) and grouped into frames. Higher-level signals are integer multiples of STS-1, creating the family of STS-N signals in Table 1. An STS-N signal is composed of N byte-interleaved STS-1 signals. Table 1 also includes the optical counterpart for each STS-N signal, designated optical carrier level N(OC-N).
TABLE 1SIGNALBIT RATE (Mbps)STS-1, OC-151.840STS-3, OC-3155.520STS-12, OC-12622.080STS-48, OC-482,488.320STS-192, OC-1929,953.280NOTE:Mbps = Megabits per secondSTS = synchronous transport signalOC = optical carrier
SONET organizes STS data streams into frames, consisting of transport overhead and a synchronous payload envelope. The overhead consists of information that allows the network to operate and allow communications between a network controller and nodes. The transport overhead includes framing information and pointers, and performance monitoring, communications, and maintenance information. The synchronous payload envelope is the data to be transported throughout the network, from node to node until the data reaches its destination.
FIG. 1A illustrates the frame format of the STS-1 signal. In general, the frame can be divided into two main areas: a transport overhead and a synchronous payload envelope (SPE). The transport overhead is three columns wide by nine rows deep, a total of 27 bytes. The SPE is 87 columns wide by 9 rows deep, a total of 783 bytes. The payload is the revenue-producing traffic being transported and routed over the SONET network. Once the payload is multiplexed into the SPE, it can be transported and switched though SONET systems without having to be examined and possibly de-multiplexed at intermediate nodes. SPEs can have any alignment within the frame. The alignment of the SPE is indicated by pointer bytes in the transport overhead. An entire STS-1 frame is transported in 125 microseconds, at a bit rate of 51.84 Mbps. The order of transmission of bits is row-by-row from top to bottom and from left to right (most significant bit first).
FIG. 1B illustrates an STS-N frame format. An STS-N is formed by byte-interleaving N STS-1 modules. The transport overhead of the individual STS-1 modules are frame aligned before interleaving, but the associated STS SPEs are not required to be aligned because each STS-1 has a payload pointer to indicate the location of the SPE or to indicate concatenation. A concatenated SONET signal, represented as STS-Nc, is a signal in which the STS envelope capacities from the N STS-1 signals have been combined to carry an STS-Nc SPE. In some systems, such as certain ISDN and ATM systems, the STS-Nc signal is used to transport data envelopes that do not fit into an STS-1 (52 Mbps) payload. To accommodate such a payload an STS-Nc module is formed by linking N constituent STS-1s together in fixed phase alignment. The payload is then mapped into a single STS-Nc Synchronous Payload Envelope (SPE) for transport. Network equipment supporting the multiplexing, switching or transport of STS-Nc SPEs treat an STS-Nc SPE as a single entity. When an STS-Nc SPE is treated as a single entity, concatenation indicators are present in the second through the Nth STS payload pointers that show that the STS-1s in the STS-Nc are linked together. Up to three STS-1 signals can be concatenated into an STS-3c. Beyond STS-3, concatenation is in multiples of STS-3c.
Established SONET networks support only STS-3c, STS-12c and STS-48c payloads (referred to as “standard STS-Nc payloads”). This leads to inefficiently mapped large payloads into SONET payloads. For example, from a bandwidth standpoint, a one Gbps (Gigabits per second, also referred to as simply “Gigabit”) Ethernet payload is preferably mapped into an STS-21c payload. The SPE of the STS-21c payload is 21×87 columns×9 row bytes every 125 microseconds (21×87 columns×9 rows×8 bits/bytes×8000 cycles/second). If sent across an established SONET network, the one Gbps Ethernet payload would be mapped into an STS-48c payload and filled with null data, not utilizing over half of the SONET bandwidth.
A solution is needed that more efficiently maps large payloads into concatenated SONET payloads while still being compatible with existing SONET network devices.